The present invention relates to aqueous dispersions of acrylic polymers for the finishing treatment of leather.
The invention specifically relates to films obtained from aqueous dispersions of acrylic polymers which confer, on the treated leather, a very good combination of softness, elasticity, abrasion-resistance, both dry and wet, resistance to water and to solvents, very good print acceptance and good adhesion to the leather substrate.
It is known, from the prior art, that the technique of finishing a leather, known as finishing, involves the use of polymer binders which are capable of forming films, which have suitable mechanical properties and which are characterized by good adhesion to the substrate. After the drying operation, the polymer film must achieve a degree of crosslinking capable of conferring, on the treated leather, characteristics of softness, of elasticity and of abrasion-resistance. This feature is particularly important in the case of leathers (splits) subjected to pressing and molding operations. In this case, it is necessary for the polymer film to withstand the print action, namely for it not to exhibit cracks and for it to be, at the same time, sufficiently plastic to retain the desired print profiles.
In the prior art, twin-component systems are used to achieve this very good combination of properties. For example, use may be made of functionalized acrylic dispersions, polyurethane dispersions, and the like, in combination with crosslinking agents, such as, for example, polyaziridines, polycarbodiimides, poly-isocyanates, and the like. The polyurethane dispersions give a very good combination of properties to the end manufactured article but exhibit the disadvantage of being too expensive. In the twin-component systems, the crosslinking agents react, generally at room temperature at the time of the formation of the film, with the functional groups present in the dispersed polymer chains. The use of twin-component systems results in the disadvantage of a very limited pot life, once the components have been mixed. Furthermore, it is necessary to stress the very high toxicity of some crosslinking agents, for example polyaziridines. These twin-component systems are described, for example, in the article in the scientific review xe2x80x9cHide Leathers Tanning Materialsxe2x80x9d, Year 68, No. 5, Sep.-Oct. 1992, entitled xe2x80x9cPrior art and possible medium-term developments of new leather finishing technologies, in particular with water-based solventsxe2x80x9d, by A. Cozzolino, pages 503-520.
An alternative to the twin-component systems in the prior art is composed of commercially available self-crosslinkable dispersions, in which the crosslinking takes place via functional groups already present in the polymer chains interacting during the phase of coalescence of the dispersion. Acrylic dispersions functionalized with N-methylolacrylamide (NMA) can be mentioned as example. The disadvantage of these dispersions is that they require a high temperature, of greater than 100xc2x0 C., in order for the crosslinking to be completed within times which are acceptable industrially. See Chemical Abstracts, 127 192133f, xe2x80x9cPreparation of an emulsion of modified polyacrylate comprising two functionalities as leather finishing agentxe2x80x9d; Chemical Abstracts, 128 90271c, xe2x80x9cPreparation of an acrylic coating resin of the MS series with interpenetrating network technologyxe2x80x9d.
Other self-crosslinkable dispersions are those comprising divalent metals, for example transition metals, disclosed in EP 789 082 and U.S. Pat. No. 5,723,782. The disadvantage of these dispersions is that they comprise a concentration of heavy metals which is often incompatible with the laws of some countries.
Self-crosslinkable aqueous dispersions based on acrylic polymers functionalized with a (meth)acrolein are also known. These dispersions are those which are the most readily used on the market, given that they make it possible to obtain polymer films characterized by a good compromise between mechanical properties, the resistance to solvents and the absence of stickiness. Even if, from the viewpoint of the final user, they do not exhibit a problem of a toxicological nature, the systems based on (meth)acrolein are difficult to handle for the producer of the polymer dispersion. (Meth)acrolein indeed exhibits the disadvantage of being a highly reactive, volatile and extremely toxic monomer, which results in problems from the viewpoint of transportation and handling.
The need was felt to have available self-crosslinkable aqueous dispersions which do not imply, for the producer of the dispersions, the disadvantages mentioned above of the (meth)acrolein-based systems but which confer, on the treated leather, a very good combination of softness, elasticity, abrasion-resistance, both dry and wet, resistance to water and to solvents, very good print acceptance and good adhesion to the leather substrate.
One object of the present invention is thus aqueous dispersions of polymers comprising the following constituents,
a) from 40 to 95% by weight of one or more monomers comprising an ethylenic unsaturation, the corresponding homopolymer of which has a glass transition temperature (Tg) of less than 0xc2x0 C., preferably of less than xe2x88x9220xc2x0 C.; the constituent a) also optionally comprising the monomers comprising the functional group defined in d), in a proportion of 20-50% by weight with respect to the amount defined in d);
b) from 4 to 58% by weight of one or more monomers comprising an ethylenic unsaturation, the corresponding homopolymer of which has a Tg of greater than 0xc2x0 C., preferably of greater than 40xc2x0 C.; (meth)acrolein being excluded from the constituent b), optionally also comprising the monomers comprising the functional groups defined in d), in a proportion of 20-50% by weight with respect to the amount defined in d);
c) from 0.05 to 4% by weight of one or more monomers comprising at least two ethylenic unsaturations;
d) from 0.2 to 5%, preferably 0.5-3%, by weight of one or more monomers comprising an unsaturation of ethylenic type and at least one functional group capable of interacting at low temperature, lower than 100xc2x0 C., in the crosslinking process in the coalescence phase, (meth)acrolein and acrylonitrile being excluded from the constituent d).
Preferably, the dispersions of the invention comprise the following constituents:
a) from 40 to 95% by weight of one or more monomers comprising an ethylenic unsaturation, the corresponding homopolymer of which has a glass transition temperature (Tg) of less than 0xc2x0 C., preferably of less than xe2x88x9220xc2x0 C.; the monomers comprising the functional groups defined in d) being excluded from the constituent a);
b) from 4 to 58% by weight of one or more monomers comprising an ethylenic unsaturation, the corresponding homopolymer of which has a Tg of greater than 0xc2x0 C., preferably of greater than 40xc2x0 C.; (meth)acrolein and the monomers comprising the functional groups defined in d) being excluded from the constituent b);
c) from 0.05 to 2% by weight of one or more monomers comprising at least two ethylenic unsaturations;
d) from 0.2 to 5%, preferably 0.5-3%, by weight of one or more monomers comprising an unsaturation of ethylenic type and at least one functional group capable of interacting at low temperature (lower than 100xc2x0 C.) in the crosslinking process in the coalescence phase, (meth)acrolein and acrylonitrile being excluded from the constituent d).
Preferably, in the dispersions of the invention, the constituent d) is added after approximately two thirds of the polymerization of the constituents a)+b)+c) with respect to the total of a)+b)+c).
Use may be made, as constituent a), of monomers of (meth)acrylic or vinyl type, and the like. Generally, the monomers of the constituent a) have from 4 to 20 carbon atoms, preferably from 5 to 15 carbon atoms. Acrylic or methacrylic esters, for example ethyl acrylate, butyl acrylate or 2-ethylhexyl (meth)acrylate, can be mentioned, for example.
Use may be made, as constituent b), of monomers of (meth)acrylic or vinyl type, and the like. Generally, the monomers of the constituent b) have from 3 to 20 carbon atoms, preferably from 3 to 10 carbon atoms. Acrylic or methacrylic esters, preferably methyl methacrylate or ethyl methacrylate, acrylonitrile, styrene or vinyl acetate can be mentioned.
Use may be made, as constituent c), of monomers comprising two ethylenic unsaturations of acrylic, vinyl or allyl type, such as, for example, allyl methacrylate or ethylene glycol di(meth)acrylate (EGD(M)A); monomers comprising more than two unsaturations of the abovementioned type are those which are preferred as constituent c), for example, trimethylolpropane tri(meth)acrylate (TMPTA).
Use may be made, as constituent d), of monomers comprising an unsaturation of ethylenic type and at least one functional group capable of interacting in the crosslinking process in the coalescence phase with the formation of covalent bonds or hydrogen bonds, preferably covalent bonds. The functional groups may be identical to or different from one another and mention may be made of the hydroxyl, amide, silane, amine and epoxide groups and the like. Mention may be made, for example, of (meth)acrylamide, hydroxyethyl (meth)acrylate, glycidyl (meth)acrylate or unsaturated silane compounds, such as, for example, alkoxysilanes, preferably comprising an unsaturation of vinyl type, where the alkoxyalkyl radical has from 1 to 10 carbon atoms, preferably more than 2. Use is made, as preferred monomers d), of those capable of giving covalent bonds, optionally in the presence of monomers capable of giving hydrogen bonds.
Use is preferably made, as constituent d), of a monomer of silane type as defined. If desired, the monomer of silane type can also be added to a combination of monomers having a different functional group.
The preferred dispersions of the invention comprise from 60 to 90% by weight of the constituent a), from 5 to 30% of the constituent b), from 0.5 to 1.5% by weight of the constituent c) and from 0.5 to 4% of the constituent d).
The dispersions of the present invention are obtained by a radical polymerization process of semicontinuous type, in aqueous emulsion, in which process a part of the monomer preemulsion prepared previously is introduced into the reactor at the beginning of the polymerization, while the remaining part of the preemulsion is subsequently introduced immediately after the exothermic peak. Generally, the exothermic peak appears after 5-20 minutes.
A preferred process for obtaining the dispersions of the invention comprises the preparation of a preemulsion composed of the constituents a)+b)+c), the introduction of 2-10% by weight of this preemulsion into the polymerization reactor to initiate the reaction; at the exothermic peak, the remainder of the preemulsion of a)+b)+c) is introduced at a constant flow rate within the space of 2 to 5 hours up to ⅔ of the final total amount; at the end, the remaining part of the preemulsion of a)+b)+c), enriched in constituent d) in the concentrations defined above, is introduced within the space of 30 minutes to approximately 2.5 hours. The feed times are, however, such that the temperature in the polymerization reactor is maintained at a predefined value.
An even more preferred process for obtaining the dispersions of the invention consists, after the initiation of the reaction with a preemulsion composed of the constituents a)+b)+c), of a sequential polymerization, first of a preemulsion of a)+b)+c) and then of a preemulsion of a)+b)+d), the preemulsions having the same ratio of the constituents a)/b); the amount by weight of the first preemulsion of a)+b)+c) representing xc2xe of the total amount of the two preemulsions and, the amount by weight of the second preemulsion of a)+b)+d) representing xc2xc of the total amount of the two preemulsions.
The initiator systems are those known in the prior art for the radical polymerization of monomers in aqueous emulsion. Mention may preferably be made of t-butyl hydroperoxide and/or persulfate salts, alone or in combination with metabisulfite salts and/or iron salts, or sodium formaldehydesulfoxylate.
The preparation of the abovementioned preemulsions is carried out with known processes, by virtue of the use of surfactants. Mention may be made, for example, of anionic surfactants, such as alkali metal salts of alkanesulfonates, for example sodium lauryl sulfate; nonionic surfactants, such as ethoxylated alkylphenols, for example nonylphenol ethoxylated with 20 mol of ethylene oxide, or ethyoxylated fatty alcohols, for example C12-C14 fatty alcohols ethoxylated with 30 mol of ethylene oxide.
The polymer dispersion of the invention can be stabilized, if necessary, with the surfactants mentioned for the preparation of the preemulsions, preferably nonionic surfactants.
The polymerization temperatures used are preferably within the range from 30xc2x0-80xc2x0 C.
When the polymerization is complete, the aqueous dispersions of the present invention in the latex form are characterized from a physicochemical viewpoint (see the examples); they are subsequently dried to give polymer films intended for the physicochemical characterization (see the examples).
As has been said, the dispersions of the invention are particularly useful in the preparation of formulations for use in the finishing treatment of leather with the optimum combination of the abovementioned properties.
The following examples are presented by way of illustration but are not limitations on the scope of the invention.